It is necessary in many electrical machines to provide an electrically conducting path between two parts which are moving relative to one another. In dynamo-electric machines, for example, it is common to use a brush of electrically conducting material sliding on the surface of a slip-ring or commutator, to provide a current path between the rotor and an external connection. A principal requirement of such a brush is that it be able to carry a high current per unit area of interface between the brush and the surface which it contacts, and it should have high wear resistance, and low friction.
Carbon, graphite, and carbon-metal blocks have been used for brushes in the past. These blocks were limited to current densities of about 100 Amp./in..sup.2, for satisfactory operation in air. With such brushes, however, typically only about 1/10,000 of the brush face surface area is available as an actual interface contact for current transfer. This is due to oxide films present in the area of interface contact, irregular brush and slip-ring surface topography, and the accumulation of surface debris. High load forces, to improve brush contact, have resulted in high brush friction and wear.
McNab, in U.S. Pat. No. 3,668,451, and Hillig, in U.S. Pat. No. 3,886,386, attempted to remedy contact problems by using multi-element brushes of encased, metal coated, tightly packed aluminum oxide or boron nitride non-conducting fibers, or elongated, plated or unplated, conducting carbon fibers. These brushes provided good contact surface area along with high strength and flexibility. They could be used for current densities on the order of about 1,000 Amp./in..sup.2, at continuous sliding speeds of up to about 18,000 ft./min.
Efforts to eliminate high wear and voltage drop due to oxide films, have included the use of hydrogen gas as a cooling medium, in conjunction with the introduction of a small quantity of mercury vapor into the non-oxidizing cooling gas, as taught by Baker et al, in U.S. Pat. No. 1,922,191. More recently, air, conditioned with alcohols, ethers, esters or ketones, has been used to cool and lubricate brushes for d.c. generators or motors, used in high altitude aircraft, and operating in dry rarefied air, as taught by Fisher et al, in U.S. Pat. No. 2,662,195, and by Savage, in U.S. Pat. No. 2,703,372.
Within the last fifteen years, a large amount of interest has been shown in the development of homopolar machines for ship propulsion or for pulse duty fusion power applications. Generally, these are machines in which the magnetic field and the current flowing in the active conductors maintain the same direction with respect to those conductors while the machine is in steady operation.
For high efficiency and acceptable machine size, the current collection systems for these high-current rotating machines must operate under very severe conditions. The current density levels at the brush interface contact may be as high as 5,000 Amp./in..sup.2, at continuous sliding speeds of up to 20,000 ft./min. Pulsed duty machinery may call for 25,000 Amp./in..sup.2 at 65,000 ft./min., at times, for hundreds of milliseconds.
British Pat. No. 1,256,757 attempted to solve current collection problems in homopolar dynamo-electric machines, by using a very sophisticated and costly liquid metal current collection system of the sodium-potassium type. While these metal type current collection systems provide high electrical conductivity and intimacy of contact, they also pose serious machine design, turbulence, toxicity and material compatibility problems.
In order for homopolar and other types of high-current electrical machines to be economically attractive, new types of current collection and environment means must be developed that are simple and inexpensive, and which keep electrical and frictional current collection losses at a minimum.